1. Field of the Invention
The present invention relates to a developer supply roller (hereinafter, sometimes referred to as “toner supply roller” or “supply roller”) in a developing device used for electrophotographic image formation process and to an image forming apparatus using a non-magnetic one component developer and the supply roller.
2. Description of the Related Art
Various types of elastic rubber rollers are used in electrophotographic copiers and printers. For example, a toner supply roller for supplying toner to a developing roller is used in a developing device in which a latent electrostatic image formed on a photoconductor is developed. A given amount of toner is supplied from the toner supply roller to the developing roller, and a latent electrostatic image is developed by the toner deposited on the surface of the developing roller. To achieve stable image formation, the toner supply roller plays an extremely important role; for example, poor supply of toner from the toner supply roller to the developing roller will result in the formation of blurred copies with reduced image density. Further, the supply roller not only functions to supply toner to the developing roller, but also to recover toner on the developing roller, which has not been subjected to development. If toner on the developing roller is not recovered but remains on the developing roller for a prolonged period of time, the toner particles are rubbed against each other many times at a developer regulation portion and, as a result, the toner shows an abnormal level of charge amount. Moreover, the toner is degraded due to stress caused by being rubbed and adversely affects image quality. Therefore, it is now strongly demanded to develop a toner supply roller, which can always supply a fixed quantity of toner and can stably recover the toner on a developing roller.
In recent years, smaller image forming apparatus and higher printing speed have been demanded. As the apparatus becomes smaller, there is a necessity for miniaturizing a developing unit, which in turn results in a necessity for manufacturing a developing roller and a supply roller smaller in terms of their diameter. Further, it is necessary to increase the rotational speed of each roller when increasing the printing speed. Accordingly, rollers with smaller diameters require increased rotational speed. In this case, toner charge amount and toner transfer efficiency decreases.
Further, in association with the miniaturization of apparatus and the necessity for greater printing speed, the supply roller is required to be made smaller in diameter and rotated at high speed. In order to retain toner chargeability and transfer efficiency and to attain a stable image output with such smaller rollers, it is contemplated to increase the penetrating amount (indentation amount) of the supply roller with respect to the developing roller. When the indentation amount the supply roller to the developing roller is increased, toner is more efficiently scraped by the developing roller. However, this results in increased frictional force between the rollers and the rotational torque of each roller increases. Moreover, the rollers with small diameters have low heat capacity and generate heat due to friction, whereby the toner is heated to elevated temperatures and is thereby firmly attached to the developing roller or regulation blade, resulting in unstable image formation. Furthermore, increasing the indentation amount of the supply roller into the developing roller gives undue stress to toner, leading to degradation of toner stored in the developing unit. Thus, there is a fear that image quality decreases when the toner is used for a prolonged period of time.
On the other hand, a method has been contemplated for decreasing the modulus of repulsion elasticity of the supply roller in order to reduce the rotational torque of the supply roller. A decrease in the modulus of repulsion elasticity of the supply roller makes it possible to reduce the rotational torque. Low repulsion foams are excellent in impact absorption and vibration absorption. Particularly, where such foams are employed as a material of a supply roller and a cleaning roller, they are in contact with a developing roller and photoconductor at a uniform contact pressure and thereby local stresses are relieved. Thus, excellent toner scraping property can be expected at low contact pressures. However, foams are less resilient after application of strain and thus it is difficult to stably form a nip between the rollers. Foams are particularly difficult to employ as a high-speed rotating body. A simple decrease in the modulus of repulsion elasticity of the supply roller will make a nip portion unstable, thereby adversely affecting the toner chargeability and transfer efficiency and cause image unevenness.
In an attempt to solve the foregoing problems Japanese Patent Application Laid-Open (JP-A) No. 2006-184602 has proposed a toner supply roller composed of a metal shaft, a first layer and a second layer, wherein the first layer (shaft side) is greater than the second layer (surface side) in average pore diameter. However, considering the fact that foams with larger average pore diameters have smaller modulus of elasticity, this proposal will result in the formation of unstable nip portion and unstable toner supply by a supply roller.
Further, JP-A No. 2006-154537 has proposed a supply roller in which an appropriate modulus of repulsion elasticity of the supply roller measured in accordance with JIS K6400 is in a range of 30% to 60%. With this proposal however, it is still difficult to make the contact pressure of the supply roller against the developing roller and photoconductor uniform for relieved local stresses, to keep the nip portion stable, and to provide stable toner chargeability and excellent toner efficiency at the same time.
Neither of the above Patent Literatures can fully solve the above problems, i.e., increased frictional resistance with an increase in the indentation amounts of the rollers, increased rotational torque, accelerated toner degradation, and heat generation from the rollers. On the other hand, where the elasticity moduli of the developing roller and supply roller are reduced, the rotational torque can be reduced. However, this raises another problem of unstable nip portion and unstable toner supply by the supply roller, leading to poor toner charge amount and poor toner transfer efficiency that may cause image unevenness.